Many gas streams contain components which are undesirable and which need to be removed from the gas stream prior to its discharge to the atmosphere or further processing. One such component is hydrogen sulfide, while another such component is sulfur dioxide.
The combustion of sulfur-containing carbonaceous fuels, such as fuel oil, fuel gas, petroleum coke and coal, as well as other processes, produces an effluent gas stream containing sulfur dioxide. The discharge of such sulfur dioxide-containing gas streams to the atmosphere has lead to the incidence of the phenomenon of "acid rain", which is harmful to a variety of vegetation and other life forms. Various proposals have been made to decrease such emissions.
Hydrogen sulfide occurs in varying quantities in a variety of gas streams, for example, in sour natural gas streams and in tail gas streams from various industrial operations. Hydrogen sulfide is odiferous, highly toxic and a catalyst poison for many reactions and hence it is desirable and often necessary to remove hydrogen sulfide from such gas streams.
There exist several commercial processes for effecting hydrogen sulfide removal. These include processes such as absorption in solvents, in which the hydrogen sulfide first is removed as such and then converted into elemental sulfur in a second distinct step, such as a Claus plant. Such commercial processes also include liquid phase oxidation processes, such as Stretford, LO-CAT, Unisulf, Sulferox, Hiperion and others, whereby the hydrogen sulfide removal and conversion to elemental sulfur normally are effected in reaction and regeneration steps.
In Canadian Patent No. 1,212,819 and its corresponding U.S. Pat. No. 4,919,914, the disclosure of which is incorporated herein by reference, there is described a process for the removal of hydrogen sulfide from gas streams by oxidation of the hydrogen sulfide at a submerged location in an agitated flotation cell in intimate contact with an iron chelate solution and flotation of sulfur particles produced in the oxidation from the iron chelate solution by hydrogen sulfide-depleted gas bubbles. In this prior art operation, both an oxygen-containing gas stream and a hydrogen sulfide-containing gas stream are distributed as fine gaseous bubbles at the same submerged location in the iron chelate solution to effect oxidation of the hydrogen sulfide.
In practice, it has been found that the quantity of oxygen required to be provided to effect substantially complete oxidation of the hydrogen sulfide to sulfur significantly exceeds the stoichiometric quantity theoretically required and experimentation has been unable to decrease the oxygen requirement below about five times stoichiometric. In other prior art hydrogen sulfide-removal processes, generally more than twenty times the stoichiometric quantity of oxygen is required.